If your vet is a very well educated avian vet, this may be where the idea came from. I seem to recall some of the bird ladies mentioning this with apple and wood from stone fruit trees. Birds are quite a bit more sensitive than piggies to some things.

We have 2 apple trees and a pear tree in our garden. Whenever we trim the trees back the piggys get loads of branches which they adore! They strip them back to nothing and they love the leaves. But like lisam mine much prefer it straight from the tree.